Flat packing and method for the production thereof

ABSTRACT

A flat gasket made up of at least one layer of nonwoven fabric consisting of a sheet of asbestos-free fibers or fiber mixtures and impregnated with a polymer, the sealing element consisting of at least one polytetrafluoroethylene-impregnated nonwoven fabric.

CROSS-REFERENCE TO-RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/EP02/02018, filed Feb. 26, 2002. This application claims the benefitof German Patent Application 10128346.6, filed Jun. 13, 2001. Thedisclosure of the above applications are incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a flat gasket, the sealing element of which isa nonwoven fabric impregnated with an active agent, particularlypolytetrafluoroethylene (PTFE), and to a process for making such agasket.

BACKGROUND OF THE INVENTION

In the (petro)chemical industry or in energy-producing and processingplants, flat gaskets made of rubber-bonded materials and of pressedsheets or pressed films of flexible graphite are often used for flangedjoints. The fillers in such cases are mostly fibrous materials, mineraladditives, graphite, charcoal, metal or metallic inserts, or appropriatecombinations thereof. The fibrous materials can also be speciallyimpregnated.

PTFE has found wide use in flat gaskets requiring very high chemicalresistance combined with good sealing and mechanical properties of thegasket material at high temperatures.

It is known from DE-A 44 19 007 to create flat gaskets made of at leastone nonwoven fabric prepreg in which a sheeting made of asbestos-freefibers or fiber blends is impregnated with a pre-crosslinkable solution,dispersion or suspension of a reactive polymer mixture, thenpre-crosslinked by drying at an elevated temperature and optionallylaminated to a plastic or metal film.

Document EP-B 654 625 discloses flat gaskets consisting of microporous,stretched PTFE that are provided at least in part with asolvent-resistant polymeric lacquer.

Document U.S. Pat. No. 5,997,008 describes a sealing ring consisting ofa corrugated metallic ring which, at least on one side, is provided witha nonmetallic coating exhibiting a low coefficient of friction, forexample with a PTFE coating, and acting as a microsealing layer.

SUMMARY OF THE INVENTION

The object of the invention is to provide a flat gasket exhibiting highsealing ability in combination with unusually good mechanical and heatcharacteristics and optimum chemical resistance.

To this end, the starting point for producing such gaskets aremechanically bonded nonwoven fabrics with an irregular or random fibrousstructure. Of particular importance is the fact that the nonwoven fabricis predominantly mechanically bonded, because the material used forchemical bonding can, during subsequent use, volatilize at hightemperatures which would result in insufficient coherence or would, forexample during subsequent use as a gasket in pipeline systems of thechemical industry, result in insufficient chemical resistance. The goodmechanical supporting action and the openness and porosity are based onthe length of the fibers that are preferably in the range from 3 to 20mm. The weight per unit is about 20 to 500 g/m². The shortness of thefibers generates high porosity that is advantageous for the subsequentimpregnation.

Predominantly used because of its good heat and chemical resistance andoutstanding mechanical strength are mechanically bonded nonwoven aramidefabrics. Nonwoven fabric, however, made of others fibers and fiberblends, for example of polyimide (PI), polybenzimidazole (PBI),polyester (PES), glass fibers (GF), aramide fibers (AF) or basalt fiberscan also be used, depending on the intended application. The bonding ofthe nonwoven fabric is done mechanically, for example, by water-jetbonding or needling. Mechanical bonding is important because, incontrast to chemical bonding, it produces high stability even at hightemperatures. The nonwoven fabric web itself has a thickness of 5 mm orless and preferably from 1 to 2.5 mm.

The great advantage of nonwoven fabric impregnation compared to, forexample, the known impregnation of woven fabrics lies in the fact thatin nonwoven fabrics, compared to woven fabrics in which the fibers arebunched together, the fibers are separated from each other and welldistributed so that each fiber is enveloped by the impregnant. In wovenfabrics or braided webs, on the other hand, impregnation takes placeonly externally, resulting in substantially less effective sealing andparticularly in less effective gas tightness.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWING

The present invention will become more fully understood from thedetailed description and the accompanying drawing, wherein:

FIG. 1 shows an apparatus for fabricating flat gaskets according to theprinciples of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiments is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses.

The quasi-continuous impregnation of the nonwoven fabric with, forexample, a PTFE dispersion containing 50 to 60 wt. % of PTFE, 3 to 5 wt.% of alkylphenol ethoxylate, <0.2 wt. % of ammoniumpentadecafluorooctanoate and the remainder demineralized water iscarried out by pulling the nonwoven fabric webs automatically through animpregnation bath at a rate of about 0.1-5 m/min. The residence time inthe impregnation bath depends on the desired degree of impregnation andon the active agent used. Predominantly used for impregnation are pureaqueous PTFE dispersions or mixtures of aqueous PTFE dispersions withmineral fillers, the mixtures containing as the inorganic filler, forexample, up to 50 wt. %, based on the dry weight of PTFE, of graphite,talc, mica or molybdenum sulfide and common dispersing aids. To makesure that the mixture in the impregnation bath is uniform, the mixturecan be agitated, for example, with an agitator or with the aid of pumps,depending on the impregnant mixture involved. In this manner, it isensured that the nonwoven fabric in question will be impregnated to themaximum degree of saturation (up to about 95-98% of active agentcontent).

In cases in which, for reasons of sealing stability, a higher proportionof fibers is desired in the finished gasket, part of the impregnant isagain squeezed out from the nonwoven fabric on a downstream squeezingsection 2. This is done also to achieve very good distribution of theactive agent in the nonwoven fabric web. To this end, the gap betweenrolls is between about 50% of the starting thickness of the nonwovenfabric and the maximum prepreg thickness.

To prevent impregnant loss by dripping, the drying of the impregnatednonwoven fabric is preferably carried out in a tunnel oven at atemperature from about 30 to 300° C.

In this manner, the dispersants and other volatile impregnationmaterials are removed so that the impregnated nonwoven fabric finallyconsists exclusively of a solid, uniform fibrous structure made upexclusively of the high-quality fibers of the nonwoven fabric and thePTFE active agent combination which even before being subjected tosintering gives a chemically and mechanically very stable flat gasket.Because of the low degree of densification, such a gasket isparticularly well suited for applications requiring high gasketdeformation, namely a still highly flexible gasket capable of adapting,for example, to sealing sites of high surface roughness or of beingused, for example, in enameled flanges or glass flanges etc.

For applications requiring higher density or gas tightness and stabilityof the flat gaskets used, in a subsequent fabrication step, the PTFEnonwoven fabric web is subjected to a heat and compression treatment bya continuous rolling or lamination process using rotating, internallyelectrically heated steel rolls. On one side, this produces markeddensification of the PTFE nonwoven fabric web and at the same time,because of the squeezing action of the rolls which have a surfaceroughness Rz of 6 μm or less, marked surface smoothing and densificationof the nonwoven fabric web takes place (smoothing or smooth squeezingand cementing of the PTFE flocks at the surface, initial sintering).This gives a smooth, gas-tight surface which is thus, among otherthings, also less prone to dirt accumulation.

If, while the web is passing through the rolls, the heat supply to thePTFE nonwoven fabric web is increased, for example by slowing down therate of web travel and at the same time raising the roll pressureexerted on the PTFE nonwoven fabric, it is also possible to sinter thePTFE nonwoven fabric web almost continuously. By applying anappropriately high pressure, it is thus possible to laminate togetherseveral (preferably 2 to 10) of these PTFE nonwoven fabric webs to anonwoven fabric structure. In this manner, for example, differentlydensified or thick PTFE nonwoven fabric webs or PTFE nonwoven fabricwebs prepared with different PTFE impregnants can be laminated together.

In particular, the lamination unit in this case consists of PTFEnonwoven fabric webs 1 wound onto width-adjusting rollers 2 with thecorresponding tensioning and breaking systems. This is followed by apreheating section 3 where a large amount of heat is supplied to thePTFE nonwoven fabric web by heating the hot-air chamber to 40-400° C.

PTFE nonwoven fabric web 1, which is to be densified and smoothed, orPTFE nonwoven fabric web 1, which is to be bonded and densified, isconveyed continuously through the calendering section by three rotatingcalender rolls 4, and passed three times between the calender rolls. Thespeed of the PTFE nonwoven fabric web is preferably between 0.1 to 2m/min, depending on the residence time desired.

To this end, rolls 4 are heated to a temperature between 30 and 300° C.

To densify PTFE nonwoven fabric web 1, namely to apply pressure, the gapbetween the metallic rolls can be chosen almost at will by making usethe nearly independent horizontal displaceability of the rolls.Preferably, the gap amounts to between 95% and 50% of the thickness ofthe undensified PTFE nonwoven fabric web 1 or to between 95% and 50% ofthe total thickness of several PTFE nonwoven fabric webs 1, saidthickness being made up of the sum of the individual PTFE nonwovenfabric webs that are to be bonded together.

The PTFE nonwoven fabric web thus densified and heated and optionallylaminated is then cooled on a cooling section and wound onto a wind-uproll.

The description of the invention is merely exemplary in nature and,thus, variations that do not depart from the gist of the invention areintended to be within the scope of the invention. Such variations arenot to be regarded as a departure from the spirit and scope of theinvention.

1. A flat gasket comprising a sealing element including at least onelayer of nonwoven fabric made of one selected from the group consistingof asbestos-free fibers and fiber blends, the nonwoven fabric beingimpregnated with a polytetrafluoroethylene (PTFE) dispersion; andwherein said PTFE dispersion includes ammonium pentadecafluorooctanoate.2. The flat gasket according to claim 1, wherein the PTFE dispersioncontains inorganic fillers.
 3. The flat gasket according to claim 1,wherein the PTFE dispersion contains as inorganic filler up to 50 wt. %of at least one selected from the group consisting of graphite, talc,mica and molybdenum sulfide, based on a dry weight of PTFE.
 4. The flatgasket according to claim 1, wherein the nonwoven fabric is amechanically bonded nonwoven fabric with a fiber length from 2 to 100mm, and a weight per unit area from 20 to 500 g/m².
 5. The flat gasketaccording to claim 4, wherein the mechanical bonding of the nonwovenfabric is carried out by water-jet bonding or needling.
 6. The flatgasket according to claim 1, wherein the nonwoven fabric comprisesfibers or fiber blends of fibers selected from the group consisting ofpolyimide (PI), polybenzimidazole (PBI), polyester (PES), glass fibers(GF), aramide fibers (AF) and basalt fibers.
 7. The flat gasketaccording to claim 6, wherein the fibers comprise aramide fibers.
 8. Theflat gasket according to claim 1, wherein the nonwoven fabric is amechanically bonded nonwoven fabric with a fiber length from 3-20 mm. 9.A flat gasket comprising a sealing element including at least one layerof nonwoven fabric made of one selected from the group consisting ofasbestos-free fibers and fiber blends, the nonwoven fabric beingimpregnated with a polytetrafluoroethylene (PTFE) dispersion; andwherein said PTFE dispersion includes alkylphenol ethoxylate.
 10. Theflat gasket of claim 1, wherein said PTFE dispersion includes 50-60 wt %of PTFE, 3-5 wt % of alkylphenol ethoxylate, less than 0.2 wt % ammoniumpentadecafluorooctanoate, and the remainder demineralized water.
 11. Aflat gasket comprising a sealing element including at least one layer ofnonwoven fabric made of one selected from the group consisting ofasbestos-free fibers and fiber blends, the nonwoven fabric beingimpregnated with a polytetrafluoroethylene (PTFE) dispersion thatincludes 50-60 wt % of PTFE, 3-5 wt % of alkylphenol ethoxylate, lessthan 0.2 wt % ammonium pentadecafluorooctanoate, and the remainderdemineralized water.